Membrane assisted continuous flow analysis was probably first described by Skeggs in 1957 in his famous paper on Segmented Continuous Flow Analysis (SCFA), Am. J. Clin. Pathol., 28, pp. 311-322. A pumped stream of liquid sample, sample conditioning liquid and air were joined to form an air segmented mixed stream that was then exposed to one side of a two-sided membrane. The other side of the membrane was exposed to an air segmented receiving stream of liquid which was subsequently directed to a flow-through detector. A component of interest in the sample permeated through the membrane under essentially steady state conditions into the liquid segments of the receiving stream to form a permeated sample component stream. Then a color forming reagent was added to the permeated sample component stream to form a colored reaction product stream which was flowed to the detector so that the concentration of the reaction product could be determined. The membrane serves an important function in membrane assisted continuous flow analysis. The membrane essentially blocks permeation of components of the sample that would otherwise interfere with the analysis, e.g., particulates in the sample. At the same time, the membrane allows permeation of a component of interest in the sample or a reaction product of the component of interest.
More recently, Flow Injection Analysis (FIA), also a continuous flow procedure but without air segmentation, has proved to be a formidable contender to SCFA (see U.S. Pat. No. 4,022,575 to Hansen and Ruzicka). In FIA, a liquid sample is injected into a flowing stream of liquid carrier. The liquid carrier usually contains a reagent so that the reagent can react with a component of the sample to form a reaction product. The flowing stream of liquid carrier is directed to a flow-through detector so that the reaction product can be determined. One advantage of FIA is that the injected sample disperses in a controlled manner with the carrier stream. Another advantage attributed to FIA is that any reaction need not be at steady state or be complete.
Membrane assisted Flow Injection Analysis is known using gas diffusion membranes, e.g., Van Der Linden, 1983, Analytica Chimica Acta, 151, pp. 359-369. In Van Der Linden's system, the sample, containing ammonia as the sample component of interest to be determined, is injected into a 0.1 Molar sodium hydroxide carrier stream and then flowed past a porous PTFE or a porous polypropylene gas diffusion membrane. The other side of the membrane is swept with a stream of 0.1 Molar sodium hydroxide receiving liquid to which stream is added a stream of Nessler's reagent prior to its flowing through a photometric detector set at 410 nanometers. Ammonia in the sample diffuses as a gas through the porous membrane to the receiving liquid and the Nessler's reagent forms a colored product when reacted with ammonia. Membrane assisted Flow Injection Analysis using a membrane where the component of interest in the sample is absorbed by the membrane has not been shown but has been suggested as will now be discussed.
Coyne et al., in U.S. Pat. No. 4,715,217, teach primarily a membrane assisted analytical chemistry method for the determination of the concentration of an organic compound in an aqueous matrix wherein the concentration of the organic compound is greater than the solubility limit of the organic compound. For this application the method of Coyne et al. is excellent. Coyne et al. taught injecting such a sample into a carrier stream containing an emulsifying agent so that the injected sample was carried past one side of a two-sided silicone rubber membrane (see Example 5). The other side of the membrane was swept with a gas receiving stream. An emulsified volatile component of the injected sample was absorbed by the membrane and then desorbed over an eight-minute time span into the gas receiving stream. The desorbed volatile component was carried to a cooled region where it was condensed and collected prior to analysis by gas chromatography. Coyne et al. stated (e.g., see column 3, lines 57-60) that the receiving stream on the other side of the membrane can be a gas for volatile sample components or a liquid for soluble sample components, and suggested (e.g., see column 4, lines 57-63) that the receiving stream could be sent directly to a detector to determine the volatile or soluble sample component that permeated through the membrane. Coyne et al. suggested that if the sample component of interest was below its solubility limit, an emulsifying agent is not necessary in the carrier (e.g., see column 1, lines 46-63). Thus, Coyne et al. also suggested a membrane assisted FIA method for determining the concentration of a sample component of interest below its solubility limit, comprising the steps of: injecting the sample into a flowing stream of liquid carrier; flowing the injected sample past one side of a two-sided membrane which absorbs a component of interest from the sample; flowing a liquid receiving stream past the other side of the membrane to a detector, so that the component of interest can be desorbed into the receiving stream and then be flowed to the detector for determination of the concentration of the permeated sample component.
Problems with this suggested membrane assisted FIA method are poor detection limits and increased analytical time due to the time needed to desorb the component of interest from the membrane with the liquid receiving stream. Coyne et al. refer to analytical times of from 8 to 20 minutes (using a gas receiving stream) but it would be preferable to have an analytical time much shorter than this and preferably less than 5 minutes.